Wireless access networks have become a key element of a variety of telecommunications network environments. As to enterprise network environments, they provide convenient wireless access to network resources for employers or customers carrying laptops and/or mobile handheld devices. In addition, wireless access points operable with diverse communication devices, such as laptops, mobile phones, etc., are broadly used in public environment such as e.g., hotels, train stations, airports, restaurants, schools, universities and homes, and are mostly used to offer high-speed internet access.
The telecommunication industries and operators are currently specifying and investigating the possibility to further increase the coverage area offered by cellular communications network systems to home or small areas. Examples of cellular communication network systems are: the Universal Mobile Telecommunication Systems (UMTS) network, also known as third generation (3G) cellular network system or wideband code division multiplexing access (WCDMA) and the Long Term Evolution (LTE) which is the next generation mobile communication system of the UMTS system. It is the 3rd Generation Partnership Project (3GPP) that is responsible for the standardization of UMTS and LTE.
According to such investigation, a limited number of users with user equipments (UE) may be provided with e.g. 3G coverage and/or LTE coverage using a small radio base station or a femto radio base station. In the 3G system, a femto radio base station is commonly called a femto NodeB or a home 3G access point (H3GAP) or a Home NodeB (HNB). In LTE, a femto radio base station is commonly called a Home eNodeB (HeNB) or a femto eNodeB. The coverage area of a HeNB or of a HNB is called a femto cell to indicate that the coverage area is relatively small compared with a coverage area of a macro radio base station. The coverage area of a macro radio base station is called a macro cell. In the 3G system, a macro radio base station is called a NodeB whereas in LTE, the macro radio base station is called an evolved NodeB or eNB.
A femto base station is configured to provide normal LTE/3G coverage for the end users in the femto cell and is further configured to be connected to a mobile operator's network using some kind of an Internet Protocol (IP) based transmission. One alternative for the IP based transmission is to use fixed broadband access (like xDSL, Cable, etc.) or broadband mobile IP based transmission to connect the femto base station to the network. The end user provided with a UE in a home environment or a in an enterprise can thus connect the femto base station to an existing broadband network in order to achieve connectivity to the mobile core network. Furthermore, these femto base stations can be activated in different locations without involvement of the operator i.e. without any operator control as end users move around.
In the LTE system, a femto base station may use the same frequency spectrum (or part of the same frequency spectrum) as the macro base station (eNB or eNodeB). This is also the case for the 3G system. If a femto base station is allocated a dedicated spectrum, the interference between the femto and macro base stations are avoided. However, it is very likely that femto base stations will share the same spectrum (or part of the same spectrum) as that of the macro base station. This implicates that the probability of interference increases. The interference is further emphasized by access restrictions to the femto cells, where only a limited number of users are allowed access. Thus if the femto and macro radio base stations share the same spectrum and they can freely allocate resources within the shared spectrum, the uplink and downlink interference may radically increases.
One way to reduce the interference when femto and macro radio base stations share the same frequency spectrum could be to allocate a part of the spectrum so that a part is primarily used by the femto physical layer and then make sure that the macro base station avoids, as far as possible, scheduling users in the part of the spectrum allocated to the femto layer. This is illustrated in FIG. 1 where it is shown a simplified exemplary shared spectrum allocation for the downlink. As depicted, part of the spectrum, 715 MHz-720 MHz is shown dedicated to the femto physical layer of the femto radio base station and the remaining spectrum. The other part of the spectrum 700 MHz-715 MHz can, in first hand, be used by the macro physical layer to schedule user data. By doing so, downlink interference can be limited. It should be noted a similar spectrum allocation is also possible for the uplink case, although not shown in FIG. 1.
A drawback with such a solution is that the Operation and Maintenance (O&M) of the macro and femto layers is handled by separate entities meaning that a static/manual configuration using O&M in the respective domain (femto domain resp. macro domain) is required which causes extra work and is error prone.
Another way to reduce the interference is to use a scheduling method known as Frequency Selective Scheduling (FSS). FSS relies on Channel Quality Indication (CQI) measurements reported by UEs to avoid scheduling radio resources or resource blocks where the UE is experiencing interference. For example, a UE located in the coverage area of a macro cell and which is not close to any of the coverage areas of the femto cells, will not experience downlink interference from femto base stations. However, when the macro base station schedules to this UE on resource blocks or radio resources used by the femto base station (or femto layer), interference is generated towards the UEs that are served by the femto base station thereby resulting in reduced downlink performance for the UEs in the femto cell.
Yet another way to reduce interference is to use a function known as a Inter-Cell Interference Coordination (ICIC) function. According to the ICIC function, information about allocated radio resources is transmitted using an interface called the X2 interface. However, the X2 interface is not present/defined for femto base stations. This means that ICIC cannot be used to mitigate interference between macro and femto base stations.